Investigators in Mexico have recovered at least 18 bodies from a mass grave near the Pacific coast resort city of Acapulco.

Authorities do not yet know if the bodies found in the grave are those of the 20 men abducted last month while visiting Acapulco from neighboring Michoacan state.

Police began digging at the site after receiving an anonymous tip, which led them to discover two bodies nearby.

An eruption of suspected drug violence has left more than 28,000 people dead across Mexico since the government began cracking down on cartels in 2006.

Meanwhile, U.S. officials said they have discovered a tunnel used for drug smuggling across the California-Mexico border. Authorities said they seized 20 tons of marijuana at a site near the tunnel, which ran more than 500 meters underground from Mexico to Otay Mesa, California.